An example of synchronic lenition in English is found in flapping in some dialects: the /t/ of a word like wait[weɪt] becomes the more sonorous [ɾ] in the related form waiting[ˈweɪɾɪŋ]. Some dialects of Spanish show debuccalization of /s/ to [h] at the end of a syllable, so that a word like estamos "we are" is pronounced [ehˈtamoh]. An example of diachronic lenition can be found in the Romance languages, where the /t/ of Latin patrem ("father", accusative) becomes [d] in Italianpadre and [ð̞] in Spanish padre, while in French père and Portuguese pai it has disappeared completely. Along with assimilation, lenition is one of the primary sources of phonological change of languages.

In some languages, lenition has been grammaticalized into a consonant mutation, which means it is no longer triggered by its phonological environment but is now governed by its syntactic or morphological environment. For example, in Welsh, the word cath "cat" begins with the sound /k/, but after the definite articley, the /k/ changes to [ɡ]: "the cat" in Welsh is y gath. This was historically due to intervocalic lenition, but in the plural, lenition does not happen, so "the cats" is y cathod, not *y gathod. The change of /k/ to [ɡ] in y gath is thus caused by the syntax of the phrase, not by the phonological position of the consonant /k/.

The opposite of lenition, fortition, a sound change that makes a consonant "stronger", is less common.

Lenition involves changes in manner of articulation, sometimes accompanied by small changes in place of articulation. There are two main lenition pathways: opening and sonorization. In both cases, a stronger sound becomes a weaker one. Lenition can be seen as a movement on the sonority hierarchy from less sonorous to more sonorous, or on a strength hierarchy from stronger to weaker.

In examples below, a greater-than sign indicates that one sound changes to another. The notation [t] > [ts] means that [t] changes to [ts].

The tables below show common sound changes involved in lenition. In some cases, lenition may skip one of the sound changes. The change voiceless stop > fricative is more common than the series of changes voiceless stop > affricate > fricative.

In the opening type of lenition, the articulation becomes more open with each step. Opening lenition involves several sound changes: shortening of double consonants, affrication of stops, spirantization of stops or affricates, debuccalization, and finally elision.

Sonorizing lenition occurs especially often intervocalically (between vowels). In this position, lenition can be seen as a type of assimilation of the consonant to the surrounding vowels, in which features of the consonant that are not present in the surrounding vowels (e.g. obstruction, voicelessness) are gradually eliminated.

Note: Some of the sounds generated by lenition are often subsequently "normalized" into related but cross-linguistically more common sounds. An example would be the changes [b] → [β] → [v] and [d] → [ð] → [z]. Such normalizations correspond to diagonal movements down and to the right in the above table. In other cases, sounds are lenited and normalized at the same time; examples would be direct changes [b] → [v] or [d] → [z].

L-vocalization is a subtype of the sonorization type of lenition. It has two possible results: a velar approximant or back vowel, or a palatal approximant or front vowel. In French, l-vocalization of the sequence /al/ resulted in the diphthong/au/, which was monophthongized, yielding the monophthong/o/ in Modern French.

Sometimes a particular example of lenition mixes the opening and sonorization pathways. For example, [kʰ] may spirantize or open to [x], then voice or sonorize to [ɣ].

Lenition can be seen in Canadian and American English, where [t] and [d] soften to a tap [ɾ] (flapping) when not in initial position and followed by an unstressed vowel. For example, both rate and raid plus the suffix -er are pronounced [ˈɹeɪ̯ɾɚ]. In many British English dialects, a different lenition that affects only [t] takes place: [t] > [ʔ] (see T-glottalization). The Italian of Central Italy has a number of lenitions, the most widespread of which is the deaffrication of /t͡ʃ/ to [ʃ] between vowels: post-pausal cena[ˈt͡ʃeːna] 'dinner' but post-vocalic la cena[laˈʃeːna] 'the dinner'; the name Luciano, although structurally /luˈt͡ʃaːno/, is normally pronounced [luˈʃaːno]. In Tuscany, /d͡ʒ/ likewise becomes [ʒ] between vowels, and in marked accents, the voiced stops /p t k/ in the same position become resp. [ɸ θ x/h].

Diachronic lenition is found, for example, in the change from Latin into Spanish, in which the intervocalic voiceless stops [p t k] first changed into their voiced counterparts [b d ɡ], and later into the approximants or fricatives [β̞ ð̞ ɣ̞]: vita → vida, lupa → loba, caeca → ciega.

A similar development occurred in the Celtic languages, where non-geminate intervocalic consonants were converted into their corresponding weaker counterparts through lenition (usually stops into fricatives but also laterals and trills into weaker laterals and taps), and voiceless stops became voiced. For example, Indo-European intervocalic -t- in *teu̯teh₂ "people" resulted in Proto-Celtic*tou̯tā, Primitive Irish*tōθā, Old Irishtúath/tuaθ/ and ultimately complete deletion in modern Scots Gaelictuath/t̪ʰuə/.[1]

An example of historical lenition in the Germanic languages is evidenced by Latin-English cognates such as pater, tenuis vs. father, thin. The Latin words preserved the original stops, which became fricatives in old Germanic by Grimm's law. A few centuries later, the High German consonant shift led to a second series of lenitions in Old High German, chiefly of post-vocalic stops, as evidenced in the English-German cognates ripe, water vs. reif, Wasser.

Although actually a much more profound change encompassing syllable restructuring, simplification of geminate consonants as in the passage from Latin to Spanish such as CUPPA > /ˈkopa/ 'cup' is often viewed as a type of lenition (compare geminate-preserving Italian /ˈkɔppa/).

Like other Western Romance languages, many varieties of Sardinian offer an example of sandhi where the rule of intervocalic lenition extends across word boundaries. Since it is a fully active synchronic rule, lenition is not normally indicated in the normal orthography.[2]

/b/

→ [β]: baca[ˈbaka] "cow" → sa baca[sa ˈβaka] "the cow"

/d/

→ [ð]: domu[ˈdɔmu] "house" → sa domu[sa ˈðɔmu] "the house"

/ɡ/

→ [ɣ]: gupu[ˈɡupu] "ladle" → su gupu[su ˈɣupu] "the ladle"

A series of synchronic lenitions involving opening, or loss of occlusion, rather than voicing is found for post-vocalic /p t k/ in much of Tuscany, in Central Italy. Stereotypical Florentine, for example, has the /k/ of /kasa/ as [ˈkaːsa]casa 'house' in a post-pause realization, [iŋˈkaːsa]in casa 'in (the) house' post-consonant, but [laˈhaːsa]la casa 'the house' intervocalically. Word-internally, the normal realization is also [h]: /ˈbuko/buco 'hole' → [ˈbuːho].

In the Celtic languages, the phenomenon of intervocalic lenition historically extended across word boundaries. This explains the rise of grammaticalised initial consonant mutations in modern Celtic languages through the loss of endings. A Scottish Gaelic example would be the lack of lenition in am fear/əm fɛr/ ("the man") and lenition in a’ bhean/ə vɛn/ ("the woman"). The following examples show the development of a phrase consisting of a definite article plus a masculine noun (taking the ending -os) compared with a feminine noun taking the ending -a. The historic development of lenition in these two cases can be reconstructed as follows:

Synchronic lenition in Scottish Gaelic affects almost all consonants (except /l̪ˠ/ which has lost its lenited counterpart).[3] Changes such as /n̪ˠ/ to /n/ involve the loss of secondary articulation; in addition, /rˠ/ → /ɾ/ involves the reduction of a trill to a tap. The spirantization of Gaelic nasal /m/ to /v/ is unusual among forms of lenition, but is triggered by the same environment as more prototypical lenition. (It may also leave a residue of nasalization in adjacent vowels.[4] The orthography shows this by inserting an h (except after l n r):

Some languages which have lenition have in addition complex rules affecting situations where lenition might be expected to occur but does not, often those involving homorganic consonants. In Scottish Gaelic, for example, there are three homorganic groups:[5]

d n t l s (usually called the dental group in spite of the non-dental nature of the palatals)

air an 'on the' (which causes lenition) → air a' chas mhòr 'on the big leg' vs air an taigh donn "on the brown house" (not air an *thaigh *dhonn)

In modern Scottish Gaelic this rule is only productive in the case of dentals but not the other two groups for the vast majority of speakers. It also does not affect all environments any more. For example, while aon still invokes the rules of blocked lenition, a noun followed by an adjective generally no longer does so. Hence:[5]

sgian-dubh 'Sgian-dubh' (sgian 'knife' + dubh '1 black 2 hidden'; sgian as a feminine noun today would normally cause lenition on a following adjective) vs sgian dhubh "a black knife" (i.e., a common knife which just happens to be black)

Though rare, in some instances the rules of blocked lenition can be invoked by lost historical consonants, for example, in the case of the past-tense copulabu, which in Common Celtic had a final -t. In terms of blocked lenition, it continues to behave as a dental-final particle invoking blocked lenition rules:[5]

In the modern Celtic languages, lenition of the "fricating" type is usually denoted by adding an h to the lenited letter. In Welsh, for example, c, p, and t change into ch, ph, th as a result of the so-called "aspirate mutation" (carreg, "stone" → ei charreg "her stone"). An exception is Manx orthography, which tends to be more phonetic, although in some cases etymological principles are applied. In the Gaelic script, fricating lenition (called simply lenition in Irish grammar contexts) is indicated by a dot above the affected consonant, while in the Roman script, the convention is to suffix the letter h to the consonant, to signify that it is lenited. Thus, a ṁáṫair is equivalent to a mháthair. In Middle Irish manuscripts, lenition of s and f was indicated by the dot above, while lenition of p, t, and c was indicated by the postposed h; lenition of other letters was not indicated consistently in the orthography.

Although nasalization as a feature also occurs in most Scottish Gaelic dialects, it is not shown in the orthography on the whole as it is synchronic (i.e., the result of certain types of nasals affecting a following sound), rather than the diachronic Irish type sonorization (i.e., following historic nasals). For example taigh[t̪ʰɤj] "house" → an taigh[ən̪ˠˈd̪ʰɤj] "the house".[3][6]

An example with geminate consonants comes from Finnish, where geminates become simple consonants while retaining voicing or voicelessness (e.g. katto → katon, dubbaan → dubata). It is also possible for entire consonant clusters to undergo lenition, as in Votic, where voiceless clusters become voiced, e.g. itke- "to cry" → idgön.

If a language has no obstruents other than voiceless stops, other sounds are encountered, as in Finnish, where the lenited grade is represented by chronemes, approximants, taps or even trills. For example, Finnish used to have a complete set of spirantization reflexes for /p t k/, though these have been lost in favour of similar-sounding phonemes. In Pohjanmaa Finnish, /ð/ was changed into /r/, thus the dialect has a synchronic lenition of an alveolar stop into an alveolar trill /t/ → /r/. Furthermore, the same phoneme /t/ undergoes assibilation/t/ → /s/ before the vowel /i/, e.g. root vete- "water" → vesi and vere-. Here, vete- is the stem, vesi is its nominative, and vere- is the same stem under consonant gradation.

Fortition is the opposite of lenition: a consonant mutation in which a consonant changes from one considered weak to one considered strong. Fortition is less frequent than lenition in the languages of the world, but word-initial and word-final fortition is fairly frequent.